Fogged,direct-positive silver halide emulsions containing strong oxidizing agents

ABSTRACT

FOGGED, DIRECT-POSITIVE SILVER HALIDE EMULSIONS ARE DISCLOSED WHICH HAVE IMPROVED SPEED VS. MAXIMUM DENSITY CHARACTERISTICS. IN ONE ASPECT, THE IMPROVED FOGGED EMULSIONS ARE MADE BY REDUCTION AND NOBLE-METAL FOGGING THE EMULSION AND THEN ADDING AN OXIDIZING AGENT, SUCH AS POTASSIUM FERRICYANIDE, TO THE EMULSION PRIOR TO IMAGEWISE EXPOSURE.

Oct.- 10, 1972 K. M. MILTON Q 3,697,281

FOGGED, DIRECT-POSITIVE SILVER HALIDE EMULSIONS CONTAINING STRONG OXIDIZING AGENTS Filed D69. 25, 1970 EFFECT OF ADDING- Ioo OXIDIZING AGENTS Q LLI LL] 0. (I)

IJJ 2 2 I [LI EFFECT OF INCREASING TIME OF FOGGING 50- O I I o I 50 2.00 2 so FOG DENSITY KIRBY M. MILTON INVENTOR.

BY AZQZ/ZW ATTORNEY United States Patent US. Cl. 96-108 14 Claims ABSTRACT OF THE DISCLOSURE Fogged, direct-positive silver halide emulsions are disclosed which have improved speed vs. maximum density characteristics. In one aspect, the improved fogged emulsions are made by reduction and noble-metal fogging the emulsion and then adding an oxidizing agent, such as potassium ferricyanide, to the emulsion prior to imagewise exposure.

This invention relates to improved direct-positive silver halide emulsions and processes for making direct-positive emulsions with improved photographic properties. In one aspect, this invention relates to a method of making fogged, direct-positive silver halide emulsions which have high-speed and high-maximum-density characteristics. In another aspect, this invention relates to an improved fogged, direct-positive silver halide emulsion wherein the grains have been fogged to a level wherein the obtainable density is approaching or beyond its maximum and then an oxidizing agent is added to the emulsion.

It is known in the art to make fogged, direct-positive silver halide emulsions such as the systems disclosed in US. Pats. 3,501,305, 3,501,306 and 3,501,307 by Illingsworth and 3,367,778 by Berriman. In certain systems where high speed is desired, the degree of fogging is carefully controlled and the monodispersed emulsions are used to obtain uniform response characteristics on each silver halide grain. However, in certain instances it would be desirable to obtain a direct-positive emulsion which has higher obtainable maximum density upon development, as well as high speed.

I have now found that high-speed, high-maximumdensity, fogged silver halide emulsions can be made by fogging the grains beyond the level which would nor mally be used to obtain a high-speed emulsion and then adding an oxidizing agent to the emulsion before the imagewise exposure. Surprisingly, I have found that the speed vs. fog density relationship does not follow an exact reverse path of that observed during the fogging step. It is apparent from the figure, which is a graphical representation of Example 2, that highly improved speed vs. maximum density are obtained when an oxidizing agent such as potassium ferricyanide is added after the fogging step and prior to the imagewise-exposure step.

The effect of adding oxidizing agents to the fogged emulsion is clearly shown in the figure which is a pictorial representation of the data from Example 2. The use of" oxidizing agents does not simply reverse the speed characteristics obtainedby increased fogging, but provides new speed vs. density characteristics which are quite desirable in certain applications of direct-positive silver halide emulsions.

In one embodiment of this invention, the direct-positive emulsions are reduction and noble-metal fogged. In another embodiment of this invention, the silver halide emulsion is preferably a monodispersed emulsion.

In a highly preferred embodiment of this invention, the silver halide grains have been reduction and gold fogged.

In another highly preferred embodiment, the useful oxidizing agent of this invention can be characterized by its ability to oxidize metallic gold or metallic silver located on the surface of a silver halide crystal.

Generally, the direct-positive emulsions of this invention are those which have been fogged to a level wherein a discernible density is produced when the emulsion is developed in a surface developer. An oxidizing agent is added to the emulsion at some time subsequent to the fogging step and prior to the imagewise exposure in accordance with this invention. Preferably, the oxidizing agent is added to the emulsion prior to coating to achieve the most uniform results; however, it can be added in some instances after coating the emulsion on a support, such as by simply bathing the coated emulsion in a solution of the oxidizing agent.

The oxidizing agents which are generally useful according to this invention are those which can lower the maximum density of a fogged silver halide emulsion coated at about 200 to about 400 mg./ft. at least 0.1 unit and preferably at least 0.2 unit. Typical useful oxidizing agents include potassium ferricyanide, potassium dichromate, sodium sulfite, sodium ferricyanide and ammonium ferricyanide. Generally, the oxidizing agent is added to the fogged silver halide emulsion in a concentration of about 1 mg. to about 1000 mg. per mole of silver (including metallic silver and silver halide), and preferably from about 10 mg. to about mg./mole of silver. The oxidizing agent can also be used in a bath for treatment of a coated emulsion prior to processing.

Typical useful direct-positive systems which can be made using the improvements of this invention include:

1) Emulsions containing fogged silver halide grains which have internal centers for the deposition of photo lytic silver such as those generally disclosed in Berriman, US. Pat. 3,367,778 issued Feb. 6, 1968, including emulsions comprising grains which have centers which promote the deposition of silver which are either sufliciently small or sufiiciently buried within the crystal as to be not accessible to initiate surface development to a visible image. Silver halide grains of this latter type can be provided by using very low concentrations of metal dopants or sensitizing agents throughout the precipitation or providing the metal dopant in the precipitation medium during the initial part of the precipitation.

(2) Emulsions which contain silver halide grains which are uniformly fogged to specific levels and contain electron-accepting compounds adjacent the grains, as described in Illingsworth, US. Pat. 3,501,305 issued Mar. 17, 1970.

(3) Emulsions comprising fogged regular grains which contain an electron-accepting compound or desensitizer adjacent the grains, as described in Illingsworth, US. Pat. 3,501,306 issued Mar. 17, 1970.

(4) Emulsions comprising monodispersed grains which have been reduction and gold fogged, as described in US. Pat. 3,501,307 by Illingsworth issued Mar. 17, 1970.

The silver halides employed in the preparation of the photographic compositions described herein include any of the photographic silver halides as exemplified by silver bromide, silver chloride, silver chlorobromide, silver bromoiodide, silver chlorobromide, and the like. Silver halide grains having a mean grain diameter, i.e., an average grain size in the range of about .01 to about 2 microns, preferably about .02 to about 1 micron, give particularly good results in reversal systems. The silver halide grains can be any suitable shape such as cubic or octahedral, but they are preferably cubic, and more preferably cubic-regular. In one embodiment, the halide of the preferred silver halide emulsions of this invention comprise less than 10 mole percent iodide. The photographic silver halides are made in accordance with this invention contain silver.

halide grains which are fogged. Fogging can be effected by chemically or physically treating the photographic silver halides by methods previously described in the prior art. Such fogging can be accomplished by various techniques such as chemical sensitization to fog, particularly good results being obtained with techniques of the type described by Antoine Hautot and Henri Saubenier in Science et Industries Photographiques, vol. XXVIII, January 1957, pp. 57-65. The silver halide grains can be fogged with the high-intensity light, reduction fogged with a reducing agent such as thiourea dioxide or stannous chloride or fogged with gold vor noble-metal compounds. Combinations of reduction fogging agents with gold compounds or compounds of another metal more electropositive than silver, e.g., rhodium, platinum or iridium, can be used in fogging the silver halide grains. The fogged silver halide grains in the direct-positive photographic emulsions of this invention give a density of at least 0.5 when developed without exposure for minutes at 68 F. in Kodak DK-SO Developer when a direct-positive emulsion containing such grains is coated at a coverage of 50 to about 500 mg. of silver per square foot of support.

The direct-positive photographic emulsions of this invention can comprise reduction and gold fogged silver halide grains, i.e., silver halide grains which are fogged with a combination of a reduction fogging agent and a gold fogging agent. The use of low concentrations of reduction and gold fogging agents in such a combination give unique fogged silver halide grains which are characterized by a rapid loss of fog upon bleaching, much less than one equivone equivalent weight of a reducing agent will reduce one equivalent Weight of silver halide to silver. To obtain the fogged silver halide grains which are charcterized by a rapid loss of fog upon bleaching, much less than one equivalent weight of reduction fogging agent is employed. Thus, less than about 0.06 milliequivalent of reduction fogging agent per mole of silver halide is employed in fogging the silver halide grains. Generally, about 0.0005 to about 0.06,

preferably about 0.001 to about 0.03, milliequivalent of reduction fogging agent per mole of silver halide is employed in fogging the silver halide grains in the practice of this invention. Higher concentrations of reduction fogging agent can result in a substantial loss in photographic speed. A preferred reduction fogging agent employed in combination with thegold fogging agent, or a compound of another metal more electropositive than silver, is thiourea dioxide which is preferably employed in a concentration in the range of about 0.05 to about 3, most preferably about 0.1 to about 2 mg. per mole of silver halide or about 0.005 to about 0.03 millimole per mole of silver halide. Stannous chloride is another suitable reduction fogging agent which is used in practicing this invention and is preferably used in concentrations in the range of about 0.05 to about 3 mg. of stannous chloride per mole of silver halide. The use of the reduction and gold fogging agents in low concentration gives direct-positive photographic silver halide emulsions exhibiting outstanding photographic speed. Examples of suitable reduction fogging agents which can be employed in the practice of this invention include hydrazine, phosphonium salts such as tetra-(hydroxy methyl)phosphoninm chloride, thiourea dioxide as disclosed in U.S. Pats. 3,062,651 by Hillson issued Nov. 6,

'4 p as disclosed in U.S. Pat. 2,521,926 by Lowe et al. issued Sept. 12, 1950, and the like. 7

The gold fogging agents employed in practicing this invention can be any gold salt suitable for use in fogging photographic silver halide grains and includes the gold salts disclosed in U.S. Pats. 2,399,083 by Waller et al. issued Apr. 23, 1946-, and 2,642,361 by Damschroder et al. issued June 16, 1953. Specific examples of gold fogging agents are potassium chloroaurite, alkali metal aurous thiosulfates, aurous sulfide, potassium aurithiocyanate, potassium chloroaurate, auric trichloride, 2-aurosulfobenzothiazole metho chloride, and the like. The concentration of gold fogging agent employed in the practice of this invention is subject to variation, but is generally in the range of about 0.001 to about 0.01 millimole per mole of silver halide. Potassium chloroaurate is a preferred gold fogging agent and is often used at concentrations of less than about 5 mg. per mole of silver halide and preferably at concentrations in the range of about 0.5 to about 4 mg. per mole of silver halide. Preferably, the direct-positive emulsions used in this mvention contain electron-accepting compounds, often referred to as desensitizers or electron traps, which. are generally compounds having an anodic polarographic halfwave potential and a cathodic polarographic potential which, when added together, give a positive sum. Typical useful electron-accepting compounds, along with methods of determining the polarographic potentials, are disclosed in the above-mentioned references on direct-positive emulsions, as well as in Illingsworth et al., U.S. Pat. 3,501,310 issued Mar. 17, 1970. An especially useful class of electron acceptors which can be used in the direct-positive photographic silver halide emulsions and processes of this invention are cyanine dyes such as the imidazo[4,5-b] quinoxaline dyes. Dyes of this class are described in Brooker and Van Lare, U.S. Pat. 3,431,111 issued Mar. 4, 1969. In these dyes, the imidazo[4,5-b]quinoxaline nucleus is attaltlcbed, through the 2-carbon atom thereof, to the methine 0 am.

The direct-positive emulsions of this invention also can contain compounds referred to as halogen-accepting or halogen-conducting compounds, especially if the halide of the emulsion is a high-chloride emulsion. Useful compounds of this type are generally characterized by an anodic polarographic. potential less than 0.85 and a cathodic polarographic potential which is more negative than -1.0. Highly preferred species of compounds of this type are merocyanine dyes having the above halfwave potentials. Typical compounds of this type, along with methods of determining the polarographic potential, are disclosed in the above-mentioned patents on direct-positive emuls3ions, as well as in Wise, U.S. Pat. 3,537,858 issued Nov.

The invention can be further illustrated by the following examples of preferred embodiments thereof.

EXAMPLE 1 A silver chlorobromide emulsion containing mole percent chloride and 10 mole percent bromide isprepared according to the following procedure.

A solution containing 4.22 moles sodium chloride and 0.47 mole potassium bromide and a solutionv containing 4.69 moles silver nitrate are added simultaneously over 50 minutes to a solution containing 200 g. gelatin and maintained at 50 C. and a pAg of 7.35. At the end of the reaction, an additional 174 g. gelatin are added, the silver halide emulsion is chill-set and thoroughly washed with chilled water. To a portion of this emulsion are added 0.3 mg. of thiourea dioxide/silver mole and it is heated for 20 minutes at 65 C. The emulsion is cooled and 0.2 mg. of potassium chloroaurate/ silver mole are added and the emulsion is reheated at 65 C. At various times, samples are withdrawn and cooled, 400 mg. of a carboxymethylthiazoline rhodanine dye/silver mole and 50 mg. of a phenyl imidazo quinoxaline-indolo carbocyanine dye/ mole are added and the portions are coated at 155 mg. of silver/ft. on acellulose acetate film support. The portion which has been gold fogged for 60 minutes at 65? C. is split into two parts. Both are coated as described but to one is also added potassium ferricyanide 'at 20 mg"./ silver the-improvement comprising a strong inorganic compound oxidizing agent which has the property of lowering the maximum density produced by said emulsion atlea st 0.1 density unit when said emulsion is coated on a support at 200 to 400 mg./ft. of support and imagewise exposed mole. The dried coatings are exposed and developed for 1 5 and developed in a silver halide surface image developand 6 minutes in an Elon-hydroquinone developer with ing composition, said oxidizing agent being added to said the following results: emulsion prior to imagewise exposure.

1' development 6' development Fog Percent Fog Percent Time or Reldensilver Reldensilver gold atlve slty develatlve sity develiogging Coating speed (Din-x.) oped speed (Dull!) oped 0' 65 Control 100 1. 70 68. 9 100 1. 95 90. 0 10 66 do 59 2.11 9o. 0 59 2.17 95. 0 65 do 47 2.18 90. 6 5a 2. 22 94. 0 to 65 do so 2. 31 92. 5 4o 2. 31 9a. 0 60' 65 Control plus potassium terrlcyanide..- 159 1. 97 85. 0 135 2.13 94. 1

It can be seen that the addition of the potassium fer- 2. In a direct-positive element comprisinga support havricyanide to the portion of emulsion which has been gold 25 ing thereon at least one layer comprising a fogged, directfogged for 60 minutes results in a fog density and amount of silver developed intermediate between that found for gold fogging times of 0 minute and 10 minutes and without the addition of potassium ferricyanide but at a considerable increase in speed. A similar result is found upon developing in Kodak D-85 Developer.

Similar results are obtained when the imagewiseexposed emulsion is contacted prior to development with an aqueous bath containing potassium ferricyanide.

EXAMPLE 2 An emulsion is prepared and reduction-gold fogged as described in Example 1. The emulsion is then finished by a heat treatment at the times indicated in the following table. To various samples of the 60-minute 65 C. are added various levels of potassium ferricyanide. The sampics are then coated on a film support at 155 mg. silver/ in, exposed in an Eastman 1B Sensitometer and developed for 1 minute in an Elon-hydroquinone developer with the following results:

It can be seen from the above table that higher speeds at corresponding maximum densities are obtained when potassium ferricyanide is added during the chemical finish.

The data from this example is represented in the figure to provide a pictorial representation of the improve- ,ments obtained by this invention.

Similar results are obtained when the oxidizing agents potassium dichromate and sodium sulfite are added to the fogged emulsion.

Although the invention has been described in considerable detail with particular reference to certain preferred embodiments thereof, variations and modifications can be effected within the spirit and scope of the invention.

I claim:

'1. In a fogged, direct-positive, silver halide emulsion,

positive silver halide emulsion the improvement comprising a strong inorganic compound oxidizing agent which has the property of lowering the maximum density produced by said emulsion at least 0.1 density unit when said emulsion is coated on a support at 200 to 400 mg./ft. of support and imagewise exposed and developed in a silver halide, surface image developing composition, said oxidizing agent being added to said emulsion prior to imagewise exposure.

3. In a process of making a fogged, direct-positive silver halide emulsion, the improvement comprising the addition of a strong inorganic compound oxidizing agent having the property of lowering the maximum density produced by said emulsion at least 0.1 density unit when said emulsion is coated on a support at 200 to 400 ing/ft. of support and imagewise exposed and developed in a silver halide surface image developing composition, said strong oxidizing agent being added to said emulsion after the fogging step and prior to imagewise exposure of said emulsion.

4. A fogged, direct-positive, silver halide emulsion as in claim 1 comprising about 1 mg. to about 1000 mg. of said oxidizing agent per mole of silver'in said emulsion.

5. A direct-positive element as in claim 2 comprising about 1 mg. to about 1000 mg. of said oxidizing agent per mole of silver in said emulsion.

6. A process as in claim 3 comprising the addition of about 1 mg. to about 1000 mg. of said oxidizing agent per mole of silver to said emulsion.

7. A fogged, direct-positive emulsion according to claim 1 wherein said oxidizing agent is an alkali metal or ammonium ferricyanide, dichromate 'or sulfate.

8. A fogged, direct-positive, silver halide emulsion according to claim 1 wherein said oxidizing agent is characterized as being capable of oxidizing metallic gold or metallic silver located on the surface of a silver halide crystal.

9. A fogged, direct-positive emulsion according to claim 1 wherein said silver halide emulsion has been chemically fogged.

10. A fogged, direct-positive emulsion according to claim 1 wherein said silver halide emulsion has been reduction and gold fogged.

11. A direct-positive emulsion according to claim 1 wherein said oxidizing agent is potassium ferricyanide.

12. A direct-positive emulsion according to claim 1 which has been chemically fogged to a level approaching or beyond the level which will produce maximum density in a silver halide surface developing composition.

13. A process according to claim 3 wherein said silver halide emulsion has been fogged to about or beyond the level wherein maximum densitj? is produced in a silver FOREIGN PATENTS halide, surfaceimage develbping'cqmpos itiofi. 3 7 7 1 i 14. ".A-pr0ce'ss according-10 "claim 3 wherein's'aid oxil dizing agent is "added 'at a *boncentr'ation" from-about'20' NORM G 'TORCHIM' l h 7, P 3 f? 5 W.-H.LQ IE,.JR., Assistant-Exammerv v a I R efe i'ence s Cited I UNITED STATES. PA'II-EINHI'S K 96-64 I 3,501,305 3/1970 Illingsworth 96108 10 

